1. Field of the Invention
The present invention relates to a semiconductor device which includes a circuit having a thin film transistor (hereinafter referred to as TFT), and a manufacturing method thereof. For example, the present invention relates to an electronic device on which an electro-optical device typified by a liquid crystal display panel is mounted as a component.
Note that in this specification, a semiconductor device refers to all devices that can function by utilizing semiconductor characteristics. An electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (with a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices and have been rapidly developed particularly as switching elements for image display devices.
Liquid crystal display devices are known as image display devices. Active matrix liquid crystal display devices have been often used because higher-definition images can be obtained as compared to passive matrix liquid crystal display devices. In an active matrix liquid crystal display device, when pixel electrodes which are arranged in matrix are driven, a display pattern is formed on a screen. Specifically, when voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, a liquid crystal layer which is provided between the pixel electrode and the counter electrode is optically modulated. This optical modulation is recognized as a display pattern by a viewer.
Silicon is mainly used as a material for an active layer of a TFT. Conventionally, a TFT has been formed using an amorphous silicon film.
In addition, a production technique where one mother glass substrate is cut into separate sections to obtain a plurality of panels so that mass production is efficiently performed has been employed. The size of a mother glass substrate is 300 mm×400 mm for the first generation in the beginning of 1990, which has increased to 680 mm×880 mm or 730 mm×920 mm for the fourth generation in 2000. Production techniques have progressed so that a number of display panels can be obtained from one substrate.
Further, as the substrate size has been increased, demands on improvement in productivity and reduction in cost have been increased.
As a TFT structure which satisfies these demands, an inverted-staggered (bottom-gate) TFT structure has been mainstream.
Reference 1 (Japanese Published Patent Application No. S58-86776) discloses a method for manufacturing a MOS transistor, in which a gate electrode is formed over an insulating substrate; an insulating film is formed over the gate electrode; and when glow discharge or arc discharge decomposition of a silane-based gas, or sputtering of silicon with the use of a gas to which a hydrogen gas is added is performed to selectively form an amorphous silicon layer over the insulating film, step-like discharge is performed in which discharge power of the glow discharge, the arc discharge, or the sputtering is low at the beginning of a discharge period and the discharge power is high in the rest of the discharge period.